1. Field of the Invention
This invention relates in general to the control of an induction motor apparatus driven from a DC power source through an inverter.
2. Description of the Prior Art
It is known to provide a closed loop control of an AC motor drive apparatus, including a three phase AC motor which is powered using a GTO based voltage fed inverter that is powered directly from a DC power source. The GTO switches in the inverter are to have a predetermined connection time between each phase of the motor and either one of the two power rails, i.e. high and low voltage. The relationship of each phase with respect to the other phases generates the needed AC waveforms in order to properly excite the motor.
For a transit vehicle, it is known that the torque output from the one or more propulsion motors is to be maintained at a particular level to accelerate or decelerate the vehicle at a predetermined rate. Due to various disturbances and unknown variables, it is known to provide a closed loop control apparatus that determines the torque output of the motor and controls the AC waveforms to that motor so as to maintain the desired torque output from that motor.
The torque output from an AC motor can be controlled by varying the slip frequency and/or the motor voltage. In a motor drive system where the input voltage source is DC, a variable voltage and variable frequency voltage source inverter can be used to vary these motor parameters.
It is known in the prior art to provide an induction motor control system including a regenerative braking mode operative with an inverter supplying the motor voltage for determining the provided brake torque, as shown by U.S. Pat. No. 3,815,002 of Clemente et al., including a different transformer winding connected in series with each phase of a three phase AC motor and having a shunt thyristor provided to short out each transformer winding during motoring operation. The braking torque is controlled by phase angle modulation of the thyristors.